Internet-Draft D. Bider
Updates: 4252, 4253, 4254 (if approved) Bitvise Limited
Intended status: Standards Track June 19, 2017
Expires: December 19, 2017
Extension Negotiation in Secure Shell (SSH)
draft-ietf-curdle-ssh-ext-info-10.txt
Abstract
This memo updates RFC 4252, RFC 4253, and RFC 4254 to define a
mechanism for SSH clients and servers to exchange information about
supported protocol extensions confidentially after SSH key exchange.
Status
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
Copyright
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Bider [Page 1]Internet-Draft Extension Negotiation in SSH June 2017
1. Overview and Rationale
Secure Shell (SSH) is a common protocol for secure communication on
the Internet. The original design of the SSH transport layer [RFC4253]
lacks proper extension negotiation. Meanwhile, diverse implementations
take steps to ensure that known message types contain no unrecognized
information. This makes it difficult for implementations to signal
capabilities and negotiate extensions without risking disconnection.
This obstacle has been recognized in relationship with [SSH-RSA-SHA2],
where the need arises for a client to discover public key algorithms a
server accepts, to avoid authentication penalties and trial-and-error.
This memo updates RFC 4252, RFC 4253, and RFC 4254.
1.1. Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.2. Wire Encoding Terminology
The wire encoding types in this document - "byte", "uint32", "string",
"boolean", "name-list" - have meanings as described in [RFC4251].
2. Extension Negotiation Mechanism
2.1. Signaling of Extension Negotiation in KEXINIT
Applications implementing this mechanism MUST add to the field
"kex_algorithms", in their KEXINIT packet sent for the first key
exchange, one of the following indicator names:
- When acting as server: "ext-info-s"
- When acting as client: "ext-info-c"
The indicator name is added without quotes, and MAY be added at any
position in the name-list, subject to proper separation from other
names as per name-list conventions.
The names are added to the "kex_algorithms" field because this is one
of two name-list fields in KEXINIT that do not have a separate copy
for each data direction.
The indicator names inserted by the client and server are different to
ensure these names will not produce a match, and therefore not affect
the algorithm chosen in key exchange algorithm negotiation.
The inclusion of textual indicator names is intended to provide a clue
for implementers to discover this mechanism.
Bider [Page 2]Internet-Draft Extension Negotiation in SSH June 2017
2.2. Enabling Criteria
If a client or server offers "ext-info-c" or "ext-info-s"
respectively, it MUST be prepared to accept an SSH_MSG_EXT_INFO
message from the peer.
A server only needs to send "ext-info-s" if it intends to process
SSH_MSG_EXT_INFO from the client. A client only needs to send
"ext-info-c" if it plans to process SSH_MSG_EXT_INFO from the server.